JSFiddle - React, Tailwind, and code Playground

HTML

<select id="dateformat" onchange="dateChange()">
    <option>mm/dd/yyyy</option>
    <option>dd/mm/yyyy</option>
</select>
<input id="date" onchange="dateChange()" />
<br/>
<span id="dateStr"></span>

JavaScript

function dateChange() {
    var e = document.getElementById("dateformat");
    var dateformat = e.options[e.selectedIndex].text;
    var myDate;
    if (dateformat == "dd/mm/yyyy") //checking the date format //
    {
        var value = document.getElementById('date').value;
        var format = value.split("/");
        myDate = new Date(format[2], format[1] - 1, format[0]);
        var day = myDate.getDate();
        var month = myDate.getMonth() + 1;
        var yr = myDate.getFullYear();
        document.getElementById('date').value = day + "/" + month + "/" + yr;
    } else {
        myDate = new Date(document.getElementById('date').value);
        var day = myDate.getDate();
        var month = myDate.getMonth() + 1;
        var yr = myDate.getFullYear();
        document.getElementById('date').value = month + "/" + day + "/" + yr;
    }
    document.getElementById('dateStr').innerHTML = myDate.toDateString();
}